1. Field of the Invention
The present invention generally relates to a non-reciprocal circuit element and, in particular, to a non-reciprocal circuit element used in a microwave band, such as an isolator and a circulator.
2. Description of the Related Art
In general, non-reciprocal circuit elements, such as isolators and circulators, have a characteristic in which a signal is transmitted in only a predetermined particular direction and is not transmitted in the opposite direction. By using such a characteristic, for example, isolators are used in a transmission circuit unit of mobile communication devices, such as car telephones and cell phones.
In such non-reciprocal circuit elements, in order to protect an assembly body of a ferrite having a center electrode formed therein and a permanent magnet for applying a direct current magnetic field to the ferrite from an external magnetic field, the assembly body is enclosed by a ring-shaped yoke (refer to International Application Publication No. 2006/011383) or a box-shaped yoke (refer to Japanese Unexamined Patent Application Publication No. 2002-198707).
However, since existing non-reciprocal circuit elements employ a ring-shaped yoke obtained by processing a soft iron or a box-shaped yoke for a magnetic shield component, the processing and assembly requires a large number of steps, and therefore, the manufacturing cost is increased. In addition, since a yoke is present around a ferrite and a permanent magnet, the outer shape of the non-reciprocal circuit element is increased in size. In contrast, if the size of the outer shape of the non-reciprocal circuit element is maintained unchanged, the sizes of the ferrite and the permanent magnet are reduced, and therefore, the electrical characteristics disadvantageously deteriorate. This is because, if the size of the ferrite is reduced, the size of the center electrode is also reduced, and therefore, the inductance value and the Q value are decreased.
In addition, since the yoke is in contact with or in close proximity to a circuit board, a floating capacitance is generated between the yoke and an internal electrode of the circuit board. Thus, a variation in the electrical characteristic of the non-reciprocal circuit element occurs. Furthermore, in the case in which a yoke made of a soft iron is soldered onto a ceramic circuit board, a heat stress acts on a soldered portion due to heat generated when the non-reciprocal circuit element operates, since the linear expansion coefficient of a soft iron is two to ten times that of a ceramic. Thus, the circuit board may curl, cracks may form in the circuit board, or the soldered portion may break. As a result, the reliability of the non-reciprocal circuit element is decreased.